diff --git a/core/config.mk b/core/config.mk
index 0e4e1fb..3289a89 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -991,11 +991,11 @@
 # For each group in BOARD_SUPER_PARTITION_GROUPS, a BOARD_{GROUP}_SIZE and
 # BOARD_{GROUP}_PARTITION_PARTITION_LIST may be defined.
 #     - BOARD_{GROUP}_SIZE: The maximum sum of sizes of all partitions in the group.
-#       If empty, no limit is enforced on the sum of sizes for this group.
+#       Must not be empty.
 #     - BOARD_{GROUP}_PARTITION_PARTITION_LIST: the list of partitions that belongs to this group.
 #       If empty, no partitions belong to this group, and the sum of sizes is effectively 0.
 $(foreach group,$(call to-upper,$(BOARD_SUPER_PARTITION_GROUPS)), \
-    $(eval BOARD_$(group)_SIZE ?=) \
+    $(if $(BOARD_$(group)_SIZE),,$(error BOARD_$(group)_SIZE must not be empty)) \
     $(eval .KATI_READONLY := BOARD_$(group)_SIZE) \
     $(eval BOARD_$(group)_PARTITION_LIST ?=) \
     $(eval .KATI_READONLY := BOARD_$(group)_PARTITION_LIST) \